Multiple digit code direction finder self-adjusting to different frequencies

ABSTRACT

A direction finding system producing a multiple digit code representing the direction of an emitted signal and including means for resolving ambiguities when the signal arrives at an angle normally giving rise to digit changeover.

United States Patent 1 3,631,496

[72] Inventors Charles Fink; i [56] I References Cited Fred R Burnhflm,t of Silver p g; UNITED STATES PATENTS [21] Appl No Marks 3,170,1352/1965 Yagelowich 343/113 x Filed %22, 1965 3,213,453 /1965 Morrison,Jr. et al 343/113 Patented Dec. 28, 1971 Primary Examiner-Rodney D.Bennett [73] Assignee Litton Systems, Inc. Assistant Examiner-Richard E.Berger Silver Spring, Md. Attorneys-Alfred B. Levine, Robert H. Lentzand Alan C. Rose [54] MULTIPLE DIGIT CODE DIRECTION FINDERSELF-ADJUSTING T0 DIFFERENT FREQUENCIES 18 Claims, 22 Drawing Figs.

[52] U.S. Cl 343/113 R ABSTRACT: A direction finding System producing amultiple Cl 1 /4 digit code representing the direction of an emittedsignal and Field of Search 343/113, including means for resolvingambiguities when the signal ar- 1 5 DP rives at an angle normally givingrise to digit changeover.

PHASING 37 RECEIVER NETWORK O VIDEO RECEIVER O38 COMPARATOR RECEIVERPatented Dec. 28, 1971 3,631,496

5 Sheets-Sheet 2 ATTENUATION O FIGJI |s0 go I Q 9 O 0 I I589 l l l ll lloll gig FIGJZ INVENTOIB CHARLES FINK FRED E. BURNHAM MAURY I. MARKSATTORNEY Patented Dec. 28, 1971 3,631,496

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Patented Dec. 28, 1971 a Z i INVENTOE CHARLES FINK FRED E. BURNHAM MAURYI. MARKS flax 12. f

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MULTIPLE DIGIT CODE DIRECTION FINDER SELF- ADJUSTING TO DIFFERENTFREQUENCIES This invention relates to antenna array direction findingsystems that are capable of substantially instantaneously determiningthe direction of a single incoming pulse by digital techniques, and isadditionally concerned with a modularly constructed system of this typewherein the accuracy or resolution can be improved by adding additionalmodular units.

Very generally according to the invention there is provided an antennaarray using one or more pairs of antennae and a digital processingsystem that processes the signals from the antennae by digital techniqueand divides the space under surveillance into a number of angularsectors with each sector identified by a digital code. When an incomingwave or pulse arrives from a particular sector, the system substantiallyinstantaneously responds by producing the digital code associated withthat sector. In a preferred arrangement the digital processing system isconstructed of substantially identical modular units, and the accuracyor resolution of the system may be progressively improved by addingadditional units.

According to additional features of the invention the system may betuned to select only given incoming frequencies or may be made selfadjusting to different frequencies by detecting the incoming frequencyand translating the digital code. Consequently the system is capable ofnot only distinguishing between a series of pulses of the same frequencyreceived from different directions but also between pulses at differentfrequency.

The mode of operation of the system essentially involves the making of aseries of comparisons of processed amplitude signals obtained from thepairs of antennas. By a first comparison it is determined that thedirection of the incoming wave or pulse falls within a broad angularsector and a first digit of the code is produced identifying this broadsector. By second and additional comparisons the incoming angle isprogressively located within smaller and smaller angular sectors and thesignificant digits of the code are produced. By correlating the variousdigits of the code, the direction of the incoming wave or pulse isdetermined within a resolution provided by the least significant digit.Consequently by adding additional modular units and making furthercomparisons, the direction is defined within the accuracy desired.

It is accordingly one object of the invention to provide such a systemthat can very rapidly determine the direction of a single pulse; aseries of such pulses; or a continuous wave.

A further object is to provide such a system that can observe space overa full 360 in any one plane or in more than one plane.

Another object is to provide such a system in modular form so that unitsmay be added as desired to improve the accuracy or resolution.

A still further object is to provide such a system in which angulardirection is very rapidly determined by digital techniques.

Another object is to provide such a system that can be adjusted torespond to any given wide band of frequencies and wherein the system canbe made self-adjusting to different frequencies.

Other objects and additional advantages will be more readily understoodby those skilled in the art after a detailed con.- sideration of thefollowing specification taken with the accompanying drawings wherein:

FIG. I is an electrical block diagram illustrating one manner ofprocessing the signals for digital comparison.

FIG. 2 is an electrical block diagram illustrating a coarse directionfinding system.

FIG. 3 is an illustration showing the angular relationship of anincoming wave or pulse to a pair of antennae.

FIGS. 4, 5, 9, I0, and 13 are polar diagrams showing the variation inamplitude of processed signals from the antennae as the direction of theincoming wave varies.

FIGS. 6 and 11 are linearly arranged diagrams showing the differenceamplitude of the processed signals for different directions of theincoming radio wave or. pulse.

FIG. 7 illustrates a typical tabulation of the output code from thecoarse system of FIG. 2.

FIG. 8 shows the arrangement of an antenna array using multiple pairs ofantennae.

FIGS. 12 and 14 are digital plots illustrating the variation in theoutput digital code as the direction of the incoming wave or pulsevaries.

FIG. 15 is a block diagram illustrating an alternative manner ofimproving the resolution of the system.

FIG. 16 is a block diagram illustrating a system using the antenna arrayof FIG. 8 and translating the digital code output to operate directionalindicators.

FIG. 17 is a graph illustrating the variation in the output withfrequency.

FIG. 18 is a block diagram illustrating a system that is selfadjustingto frequency variations;

-FIG. 19 is an electrical schematic diagram illustrating the connectionof directional indicators to the system.

FIG. '20 is an electrical schematic diagram illustrating circuit detailsof a preferred single modularunit according to the invention.

FIG. 21 is a three-dimensional diagram illustrating the application of asystem to surveillance of all three dimensions in space.

FIG. 22 is an electrical schematic diagram illustrating a system forobserving over a full 360".

FIGS. 1 and 2 illustrate one preferred coarse direction finding systemaccording to the invention for responding to an incoming wave or to asingle pulse received over any angle within a spatial sector of 180 andproducing a two-digit binary code defining the direction of the incomingwave within one of four angular sectors.

As shown, this coarse direction-finding system generally comprises apair of spaced antennas l0 and 11 for receiving the waves and beingsuitably focused by a lens or reflector (not shown), a phasing network60 for processing the signals from the pair of antennae, and a digitalamplitude comparison system for comparing the amplitude of the processedsignals to produce a two-digit code representing the direction of theincoming wave.

The phasing network 60 produces four processed signals. The firstproduced over line 35 is the sum of the antenna signals and is obtainedby combining the signals in an adder circuit 28. The second processedsignal produced over line 36 is the difference of the antenna signalsand is obtained by reversing the phase of the signal from antenna 10 bymeans of phase shifter 29 and adding the shifted signal with that ofantenna 11 in the adder circuit 30. The third signal is obtained byphase shifting the signal with that of antenna 11 in the adder circuit30. The third signal is obtained by phase shifting the signal fromantenna 10 by by a phase shifter 31 and adding this phase shifted signalto that of antenna 11 in adder circuit 32. And the fourth process signalis obtained by oppositely phase, shifting the antenna signal fromantenna 10 in the phase shifter 33.and adding it to thatof the antenna11 in adder 34. I

These four process signals are individually detected by receivers 39,40, 41, and 42, to remove the radiofrequency carriers and produce thevideo or envelope signals, representing the absolute amplitude of thesereceived antenna signals. These amplitude signals are then compared toobtain the two digit code representing the direction of the incomingwave. As shown in FIG. 2, the amplitude of the sum signal 35 is comparedwith that of the difference signal 36 in comparator 43 and a morepositive signal (binary 1) is produced if it exceeds this signal and amore negative signal (binary 0) is produced if it is less than thedifference signal. The processed signal including the 90 phase-shiftedcomponent is compared in amplitude with the processed signal includingthe minus 90 phase-shifted component, and similarly a binary l isproduced at the output of comparator 44 where one signal exceeds theother, and a binary is produced when the reversed condition prevails.

For an understanding of the manner wherein these compared amplitudesignals determine the angle of the incoming wave, reference is made toFIGS. 3, 4, 5, and 6.

As shown in FIG. 3, an incoming wave or pulse having a wavefront 12reaches the pair of spaced antennas l0 and 11 at different timesdepending upon the angle of approach of the wave referenced to thecommon axis of the antennae. The amplitude of the received signals atboth antennae is substantially the same but the short time delayproduces a relative phase variation in the signal at the two antennae.Due to this phase shift, the sum of the two signals has an absoluteamplitude that varies with the angle of the incoming wave as shown inthe solid line polar plot pattern 13 of FIG. 4 and the difference inamplitude of these two signals differently varies with the angle asshown by the twin dotted line patterns 14 and 15 in FIG. 4. From thesetwo patterns it is seen that the amplitude of the dif ference signalalways exceeds the amplitude of the sum signal when the radio wave isreceived from spatial angles of from 0 to 60, and from spatial anglesextending from 120 to 180, but conversely the amplitude of the sumsignal always exceeds that of the difference signal where the wave isreceived from spatial angles extending from 60 to 120. Consequently,when the output of comparator 43 in FIG. 2 produces a more positivesignal or a binary 1, it is known that the incoming wave is beingreceived from the spatial sector of from 60 to 120. On the other hand,when the comparator 43 produces a binary 0, it is known that theincoming signal is being received from either the first spatial sectorof 0 to 60, or from the third sector from 120 to 180.

To resolve this latter ambiguity, two additional synthetic lobe patternsare produced by phase shifting one of the antenna signals by 90 inopposite directions and adding each of these shifted signals to theother antenna signal. The additional synthetic lobe patterns producedfrom the processed signals using the phase-shifted components are shownby the polar plot of FIG. the patterns produced using the positivelyphase-shifted signal being indicated by the solid line pattern 18 and19, and the pattern using the negatively phase-shifted signal beingshown by the dotted lines 16 and 17. Interpreting this latter polarplot, it is noted that the amplitude of the processed signal using thenegatively shifted component 16 always exceeds the amplitude of thesignal using the positively shifted component 18 where the radio beam isbeing received from a spatial angle from 0 to 90. On the other hand, theopposite condition prevails where the radio beam or wave is receivedfrom an angle of 90 to 180. Thus by means of the second amplitudecomparison performed by comparator 44, the ambiguity arising from thefirst comparison is resolved.

FIG. 6 illustrates by means of a linear-type plot the variation inamplitude of the greater of the processed signals for waves received bythe antennas at any angle between 0 to 180. The upper curves 20 and 21illustrate that for received angles of 0 to 90, the amplitude of theprocessed signal using the negatively phase-shifted componentpredominates over the positively shifted component to produce a negativeoutput at comparator 44 (binary 0), and for angles between 90 and 180the processed signal using the positively phase-shifted componentpredominates to produce a more positive output at comparator 44(binary 1. The lower curves 22, 23, and 24 show that at angles of thereceived wave between 0 and 60, and between 120 and 180, the amplitudeof the difference signal (binary 0) predominates whereas for angles ofthe incoming wave within the sector of 60 to 120, the amplitude of thesum signal is greater (binary 1). Thus by correlating the amplitudecomparisons from the two pairs of processed signals, the direction ofthe incoming wave can be determined within one of four angular sectorsof 60 or 30. For example, it is seen that for an output binary code of00, the direction of the incoming wave is indicated to fall within theangular sector of 0 to 60. For an output code of 01, the wave is beingreceived from the angular sector from 60 to for the code 11, the wave isbeing received in the angular sector from 90 to andfinally for the code10, the wave is being received in the angular sector from 120 to Sincethis system operates substantially instantaneously and the antennaeobserved over a full 180 in space, the system can almost instantaneouslydetermine that a plurality of waves are being received at differenttimes from different directions and can specify by code, the directionsof these difl'erent waves. An example of thisv is shown in FIG. 7illustrating a typical recorded tabulation of a series of codes producedby successively received waves. As shown, the first three recorded codes(binary 10) indicated by the number 48, shows that a wave is beingreceived from an angular sector from 120 to 180. The next recorded code(binary 01) indicates that a different wave is being received from adifferent direction within the sector of from 60 to 90.

To more accurately determine the direction of the incoming wave withinprogressively smaller or narrower angular sectors, a plurality of pairsof antennas may be used inv a similar antenna array with each additionalpair of antennas being spaced more widely apart than the preceding pair.As will be seen, the further apart the pairs of antennas are spaced, thegreater is the phase displacement of the signals received, and, in turn,the greater will be the number of synthetic directional lobes producedfor defining the direction of the incoming wave. For producing the polarlobe plots of FIGS. 4 and 5, the antennas are spaced apart approximatelyby one-half wavelength of the incoming wave frequency. FIG. 8illustrates an antenna array using three pair of antennas; the firstpair 10 and 11 being spaced apart by approximately one-half wavelength,the second pair 50, 51 being spaced apart by one wavelength, and thethird pair 52, 53 being spaced apart by two wavelengths.

As is seen in the polar plot of FIG. 9, the second pair of antennas 50,51 (spaced apart by one wavelength) produces three synthetic directionallobes 54, 56, and 58 covering the variation in amplitude of the sumsignals, and two additional lobes 55 and 57 (in dotted lines) showingthe variation in amplitude of the difference signal for different anglesof the incoming wave. As seen in FIG. 10, the more widely spaced apartantennae 52 and 53 on the other hand produce four directional lobeshaving narrower angular sectors 59, 61, 63, 65, and 67 for the sumsignals, and four additional lobes 60, 62, 64, and 66 (in dotted lines)for the difference signals.

The greater of the amplitudes of the sum and difference signals for eachspatial angle of the incoming wave obtained from the antennas 50 and 51is shown in the upper linear plot of FIG. 11, and it is seen that thesignals from this pair of antennas divides the spatial angle of theincoming wave into five sectors of smaller angles. The lower portion ofFIG. 11 illustrating the greater amplitude of the sum and differenceamplitude signals from antennas 52 and 53, shows that the third pair ofantennas divides the spatial angle of the incoming wave into nineangular sectors of even smaller angle than the second pair of antennas.Thus it is seen that the digital comparisons obtained from theadditional pairs of antennas 50, 51 and 52, 53 serve to further defineor resolve the direction of the incoming wave with a progressivelynarrower angular sector.

The digital output of these comparator circuits can be represented evenmore simple than as shown in the linear plots of FIG. 11 by thedigital-coded-type angular plot shown in FIG. 12. Here the upper angularcode corresponds to the upper linear plot of FIG. 11 and the lowerdigital plot corresponds to the lower linear plot of FIG. 11. Thesecoded plots show only the greater amplitude of the compared processedsignals for each pair of antennas and represent the greater amplitude asa binary 1 if one of the amplitudes exceeds the other and by binary 0 ifthe reverse condition prevails. Referring to the upper digital plotobtained from antennas 50, 51, it is noted that the pair of antennas 50,51 divides the 180 spatial sector into five angular sectors, and thelower coded plot obtained from the more widely spaced apart antennas 52,53 divides the spatial sector into nine smaller angular sectors,overlapping those of first series. It will also be noted from FIG. 12that at a number of different angles of the incoming wave, the codeproduced by comparison of the sum and difference amplitudes-is the same,e.g. at 0, 90, and 180. Therefore it is evident that the amplitudecomparisons provided by the more widely spaced apart antennas producenarrower synthetic lobes to improve resolution but by themselves are notsufficient to resolve ambiguities and define the direction of theincoming wave. However, in combination with the digital plots obtainedfrom the most closely spaced antennas l0 and 11, it will be seen thatthese ambiguities are resolved and the incoming wave can be definedwithin the angular sector of the closest or least significant digit.

FIG. 13 shows a combined polar plot of the processed amplitude signalsobtained from all pairs of antennas, obtained by combining the polarplots of FIGS. 4, 5, 9, and 10, and FIG. 14 is a linear coded plot,corresponding to the polar plot of FIG. 13, and illustrating the mannerin which a system using all three pairs of antennas identifies thedirection of the incoming wave within a resolution or accuracy that itmany times greater than that of the coarse system.

As shown in FIG. 14, the incoming wave is digitally defined within anangular sector of about 60 by processing the signals obtained from themore closely spaced antennas l0 and 11. The uppermost digital plot,using the 90 phase-shifted component from antennas 10, 11 produces thefirst binary digit of the code indicating that the incoming wave isbeing received over the sector of from 0 to 90 (binary O) or from thesector of 90 to 180 (binary l). The next digital comparison using thesum and difference signals from antennas l0, 11 serves to further dividethe 180 spatial sector into three equal sectors of about 60. Thus bycorrelating the binary code produced by these first two digits, thedirection of the incoming wave is defined with an angular sector of 60.The third and fourth codes obtained from the antennas 50, 51 and 52, 53,respectively, further divide these angular sectors to more accuratelyidentify the direction of the incoming wave. Thus, for example, wherethe output binary code obtained from the system is 0011, it is knownthat the direction of the incoming wave lies in the angular sector fromabout 0 to 30 or where the output binary code is 1011, it is known thatthe direction of the incoming wave is from the angular sector of 150 to180.

The cross-hatched areas shown in FIG. 14 illustrate those angularsectors where the amplitudes of the processed signals being compared areso close to one another that it is difficult to determine which of thetwo amplitudes is greater. This is observed by returning to the linearplots of FIG. 11 which i1- lustrates the amplitude difference of thecompared signals from the antennas 50, 51 and 52, 53, for differentangles of the incoming wave. As shown by the upper plot in FIG. 11, thegreater in amplitude of the processed signals from the antennae 50, 51is maximum at the angles of 0, 60, 90, 120 and 180, whereas theamplitude difference of these processed signals is close to a null atangles represented at a, b, c, and d; which amplitude is three decibelsbelow the peaks. Consequently for incoming waves at angles where thesenull or lower amplitude positions occur, the binary digit produced isambiguous and may be either a binary l or binary 0. Returning to FIG.14, it is noted that these ambiguities do not overlap one another, andtherefore by using a total of only three of the digits to define thedirection of the incoming wave, the ambiguities can be resolved to onlythose that occur in the least significant digit (from antennas 52, 53).For example, considering the spatial angle of 60 or that of 120', itwill be noted that at both of these angles there is an ambiguityoccurring in the second digit. However, since there is no such ambiguityin the first, third or fourth digit, the correct angles can be definedby these three other digits neglecting the ambiguous digit. Thus theangle 60 is defined by the three-digit binary code 0-01, and the angle120 by binary code 1-01. In a similar manner where an ambiguity existsin the first or third digit, that ambiguity can also be ignored and theangle can be defined by a code consisting of the other three digits.This is not true of the last digit, and therefore the ultimateresolution or accuracy is determined by the last or least significantdigit in the code. On the other hand by adding additional pairs of morewidely spaced apart antennas and making further digital comparisons ofprocessed signals, the ambiguities in the fourth or lower digits can besuccessively resolved and the angle of the incoming wave can be moreaccurately defined within progressively narrower angular sectors asdesired.

FIG. 15 illustrates a different manner of improving the accuracy orresolution of the system using only a single pair of antennas. In thisembodiment, the signals from antennas 10 and 11 are processed in thesame manner as in FIGS. 1 and 2 to derive the sum and difference signalsand the processed signals including the phase-shifted components. Theseprocessed signals are compared in amplitude as in FIG. 2, to obtain thefirst two digits of the binary code as described above. However insteadof adding additional pairs of more widely spaced antennas to derive theadditional digits, the signals from antennas 10 and 11 are eachindividually multiplied in frequency by frequency multipliers 61 and 62and these frequency multiplied signals are processed to obtain the sum,difference, and 90 phase shifted components as before to produceadditional directional lobes. Each of these latter signals are alsodigitally compared in amplitude as before to yield the desiredadditional digits that more accurately define the direction of theincoming wave.

The theory of operation of this embodiment is similar in some respectsto that of the first embodiment due to the fact that multiplying thefrequency of these signals multiplies the phase difference of thevoltages induced in the antennas by the incoming wave. Thus if thefrequency of the antenna signals is doubled, it is the same in effect asdoubling the spacing between the antennas which would also double thephaseshift difference. It will be recalled that the amplitudecomparisons are made of the video or detected signals to derive thedigital code, and therefore the frequency multiplication of the signalshas no effect upon the digital processing since the carrier frequenciesare removed before the amplitude comparisons are made.

To improve the accuracy even further, the antenna signals may be furtherfrequency multiplied by using additional frequency multiplier stages toquadruple or otherwise increase the relative phase angle of the signalsobtained by the antennas. This is similar in effect to adding stilladditional pairs of antennas that are spaced more widely apart as in thearray shown in FIG. 8. Thus a highly accurate system can be constructedusing only a single pair of antennas together with a plurality of stagesof different ratio frequency multiplication instead of using pluralpairs of antennae for the same purpose as shown in FIG. 8. However in apractical system, the signalto-noise ratio decreases for each additionalfrequency multiplication, and the complexity and cost of the frequencymultiplying circuitry and components may be greater than the cost ofusing additional antenna elements. Consequently, the choice of usingadditional pairs of antenna or frequency multiplication stages in aparticular system will depend upon the strength or power of the incomingwave being received together with the considerations of the cost,complexity, and size of the system components for processing thesignals.

FIGS. 16 and 19 illustrate a digital direction-finding system usingthree pairs of antennae, and additionally illustrate the coupling ofthis digit system to selectively operate a series of indicators each ofwhich displays the direction of the incoming wave within a given angularsector. A system of this type for directly indicating the angle of theincoming radio wave is particularly advantageous for portabledirection-finding systems where it is desired to substantiallyinstantaneously detect and display the direction of a stationary ormovable source of radio waves, pulses, or a single pulse.

As shown, the signals from each pair of antennae are processed anddetected as described above, in what may be termed the RF sections 75,101, and 87, respectively, and the amplitudes of these selected pairs ofprocessed signals are compared in comparator circuits 43, 44, 81 and 84,as also described above, to obtain the difierences in amplitude andprovide the code. These signals from comparators 43, 44, 81, and 84 arethen directed to decision elements 76, 77, 82, and 85, respectively,which supply code pulses to suitable storage elements 78, 79, 83 and 96,respectively, for storing the output code. For selectively energizingthe proper one of a series of direction indicators, the stored code istranslated by means of a matrix, generally indicated by block 80 in FIG.16 to provide a single output pulse or signal to the appropriate one ofa series of output lines 90 to 98 leading to individual indicators.

As shown in FIG. 19 the decision elements may comprise conventional-typegate circuits that are opened and closed by an external clock or syncsource (not shown) over line 108 to direct the DC output from thecomparator circuits to conventional flip-flop circuits that serve as thestorage elements. These flip-flops are triggered in one direction or theother depending upon the output of the comparator circuits to store abinary l or a binary 0. A matrix connection couples the outputs ofpreselected ones of the flip-flops to energize the appropriate ones ofthe indicator lights 90a to 98a according to the stored binary code. Asshown, the matrix may be of conventional construction andinterconnecting the flip-flops, to the indicators through AND-gates 107.Each of the indicators which is connected to an associated AND-circuit107 which in turn responds to selected ones of the four pairs of outputlines from the amplifiers 106. The connections are made such that eachindicator will respond to a different predetermined code stored in thefour flip-flop circuits and be energized only when the preselected codeis received on all of its three AND circuit lines. Each of theseindicators is suitably marked to display the direction of the incomingwave.

In a portable direction finding system, the antennas and indicators maybe supported on a movable housing which can be manually or otherwisepositioned to scan in different directions. In a preferred constructionthe indicators are arranged in spaced relationship across the housing soas to immediately reveal the direction of the incoming wave.

A portable system embodying the present invention has been constructedthat is very small in size, light in weight, inexpensive, and usingprefabricated modular unit circuitry of a conventional nature. Furtherreductions in size and weight can be obtained using hybrid or integratedcircuits. One channel of this preferred signal processing circuitry isshown in FIG. 20 to illustrate the simplicity of this system. As shown,the sum and difference signals over lines 35 and 36 are detected bysimple crystal diodes 110 and 1 11 to produce the video or DC amplitudesignals, and these are compared in opposition by a simple resistornetwork to produce the difference signal. Most specifically, the crystaldiodes 110 and 111 are disposed in oppositely poled relationship withrespect to the sum and difference signals whereby the detector 110passes only a positive going envelope or video signal and detector 11 1produces only a negative going video signal. These positive going andnegative going video signals are directed through adjustable resistors112 and 113, respectively, interconnected in a summing circuit withresistor 114. When the amplitude of the positive going video signalpredominates over the negative going video signal, a more positivepotential is produced across resistor 114 and where the oppositecondition prevails, a more negative potential is produced acrossresistor 114. The more positive potential represents a binary 1condition and the more negative potential represents a binary condition.This difference signal is amplified by means of amplifier 115 anddirected to a Schmitt trigger circuit 116 that produces and amplifiedsquare wave pulse of greater or lesser amplitude representing the binaryl or binary 0 condition, and this square wave is thence directed to theinput of gate circuit 76. When the gate is operated by a pulse at binary1 condition, a pulse is directed to one of the input lines of theflip-flop 78, and a pulse at binary 0 energizes the other line offlip-flop 78. A pulse having a binary l amplitude condition thereforetriggers the flip-flop in one direction, and a pulse at binary 0amplitude triggers the flip-flop in the opposite direction. The outputof flip-flop 78 is directed through amplifier 106 and the matrix 80 andtogether with the energization from the other channels energizes theappropriate one of the indicators a to 98a as described above inconnection with FIG. 19. In the preferred system, the storage flip-flops78 retain the binary code identifying the direction of the last-receivedincoming radio wave until changed by the next succeeding pulse or wave.For operating the gate and flip-flop, there is provided an additionalantenna 117 and a crystal receiver 118 for detecting incoming radiowaves or pulses and producing a control video signal that is directedthrough amplifier 119 and Schmitt trigger 120 to produce an amplifiedcontrol pulse for each incoming wave or pulse received by the antenna117. This square pulse generated by the Schmitt trigger 120 operates aone-shot multivibrator 121 to produce a pulse for triggering theflip-flops into the binary I or binary 0 condition as determined by theoutput of the associated Schmitt trigger circuits. In this manner, thesystem can respond to and can indicate the direction of only a singleincoming pulse or a succession of pulses, and it retains the digitalinformation or display representing the direction of this pulse until itis changed by the next succeeding incoming wave or pulse received by theantennas. The antenna 117 may be a separate antenna in addition to thoseprovided in the array or it may be one of the antennas in the array thatis also used for this control function.

In the system described above, a single antenna array is used tomaintain surveillance over a spatial angle of 180 in one plane. In thiscase, the antenna elements used are made directional to observe only thedesired area, or alternatively suitable lenses or reflectors areemployed to focus the array. Where it is desired to observe over a full360 in any one plane, a pair of systems may be used as shown in FIG. 22.In this case, the first array 123 observes over a full 180 in onedirection and is provided with a digital processing system 124 toproduce the digital code identifying the direction of any radio signalsobserved within the surveyed area, and the second array 125 observesover the remaining and is likewise provided with a digital processingsystem 127 to produce the identifying code. Both digital processingsystems jointly feed a matrix 128, that may be similar to that of FIG.19, to energize the appropriate one of a series of indicators 129 foralmost instantaneously displaying the direction of an incoming radiopulse from any direction over the 360 observed area. As shown, areflector 126 may be used to separate the arrays and focus each on itsobserved area.

Direction finding in more than one geometric plane may likewise beobtained by using a mattress array of antenna elements in the samegeneral manner with the signals from each linear array being digitallyprocessed as described to produce its digital code and with the codesbeing combined in a matrix to energize appropriate indicators forinstant display, or being otherwise digitally processed or recorded.FIG. 21 illustrates one manner of direction finding in three geometricplanes using an antenna array 120 disposed along the X-X axis, a secondantenna array 121 along the Y-Y axis, and a third array 122 along the2-2 axis. By providing three double pairs of mattress arrays (not shown)along three orthogonal axis, three dimensional space may be fullyobserved and the direction of any incoming radio pulse identified.

VARIATION OF THE CODE WITH FREQUENCY The above described mode ofoperation applies where the frequency of the incoming radio pulse orwave is known, and the pairs of antennas are spaced apart at a givennumber of wavelengths of this known frequency as shown in FIG. 1 and inFIG. 8. If the frequency of the incoming radio pulse is higher or lowerthan that for which the system is designed, the comparative phase shiftobtained from the antenna signals is respectively greater or lesser, andthe lobe patterns of FIGS. 4, 5, 9, 10, and 13 are displaced to vary theoutput code. Consequently although the output code is correct at thedesigned 9 frequency it is incorrect at other frequencies, and thegreater the frequency deviation, the greater will be the error.

It has been found, however, that at any given angle or direction of theincoming radio wave referenced to the axis of the array, that the outputcode varies linearly with change of frequency, and therefore havingknowledge of the frequency of the radio pulse, the code can becorrected. A plot of the linear variation of the output code withfrequency is shown in FIG. 17. Referring to FIG. 17 and taking as anexample on incoming radio a pulse arriving at an angle of 50, it is seenthat this incoming radio pulse results in an output code produced (orbeam number) of 24 at the designed frequency of the system but theoutput code number linearly increases with increase in frequency suchthat at a frequency of about 1.6 times the designed frequency, the codenumber produced is 32. In a similar manner it is observed that the codenumber linearly increases with increase in frequency for incoming pulsesat all other angles except for those arriving at an angle of 90 to thearray. In this latter case, the signals received by all antennas is inphase. The variation of the code or code number with frequency can beshown to be as follows:

N Cos Where N is the code or code number,

f is the frequency for which the system is designed,

0 is the angle of the incoming radio pulse referred to the array, asshown in FIG. 1, and

K is one-half the total number of sectors within 180 at f,,.

To correct for this error with frequency, the output code produced bythe system can be manually translated using a nomograph like that ofFIG. 17 which is for a 64 sector system, or this may be performedautomatically as shown in FIG. 18. In this embodiment, the frequency ofthe incoming wave or pulse is detected by means of a frequency detector70,

of any well-known type, and a signal representing this frequency isdirected to a signal calculator 71. Concurrently the antenna signals areprocessed as before through phasing network 60, receivers 39 to 42, andcomparator-storage devices 43, 44 to derive the digital output code.This digital output code is then directed to the signal calculator 71where it is translated to correct the code for that frequency. As willbe noted from the above formula, the correction being made involves asimple mathematical operation since the variation in code number withfrequency is linear. Therefore the signal calculator 71 may employconventional circuits for this purpose. The translated or corrected codefrom the multiplier 71 is then directed to suitable storage or displaydevices 72 in the same manner as discussed above to almostinstantaneously display or record the direction of the radio pulse orwave.

As is believed now evident, the direction finding system of the presentinvention is capable of many changes and variations. The system ispreferably constructed in modular form modularly constructed digitalindependent processing system, as indicated in FIG. 20, so thatadditional pairs of antennas and associate modules may be added to anddeleted to meet frequencies as shown in FIG. 20, or more sophisticatedsuper-' heterodyne automatic band scanning devices. For sonic,supersonic, or light frequencies other types of transducers andreceivers will be used. The antennas used will also vary according tothe frequency to be detected and the specific components used willdetermine the frequency bandwidth, total angular coverage, andsensitivity. The antennas may in many with each pair of antennas beingprovided with an identical.

instances be simple monopoles over a ground plane, or planar spirals, orparabolic dishes. Components of these types are well known to thoseskilled in the art and further descriptions ,jare not considerednecessary. Since these and many other variations may be made withoutdeparting from the spirit and scope of this invention, this inventionshould be considered as fbeing limited only by the following claims:What is claimed is:

l. A direction of finding system comprising: a pair of spaced :antennas,and a rapid digital processing system responsive to the signals from theantennas to provide a digital code representative of the angle betweenan incoming wave referenced to a plane containing said pair of antennas,said digital processing system including a first decision making meansfor determining whether the sumof the signals from the antennas isgreater or less than the difference between said signals and producing adigital indication, and including a second decision making means,including phase-advancing and -retarding means for one of said signals,for determining whether the sum of the other signal and thephase-advanced 1 signal is greater or less than the sum of the othersignal and the [phase-retarded signal and producing a second digitalindication.

2. A digital beam direction-finding system comprising: a "pair of spacedantenna elements, means for adding the signals 'from said elements,means for differencing the signals from said elements, means forcomparing the amplitude of said =added signals with the amplitude ofsaid difference signals and ,producing a digital code indicative of thegreater amplitude, l means for phase advancing the signals from one ofsaid elements, means for equally phase retarding the signal from the 3same element, means for separately adding the signal from the otherelement with each of the phase-advanced and -retarded signals to producefirst and second processed signals, means for comparing the amplitudesof said first and second processed signals and producing a seconddigital code indicaztive of the greater amplitude, and means responsiveto the first and second digital code to determine direction.

3. A digital direction finding system having two displaced ;antennaelements responsive to an incoming beam comprising: means for comparingthe amplitude of the sum of signals from said elements with theamplitude of the difference of the signals from said elements andproducing a digital code indicative of the greater amplitude, means forequally phase-advancing and -retarding the signal from one of saidelements, means for separately adding the signal from the other elementwith each of the phase-advanced and phase-retarded signal to obtain afirst and second processed signal and producing an additional digitalcode indicative of the greater amplitude, and means responsive to saiddigital code and additional digital code to determine the direction ofthe beam.

4. A digital system for determining the direction of an incoming wavereferenced to an antenna array comprising: a first pair of spacedantenna elements in said array, digital decision making means responsivesolely to the comparative amvplitudes of the signals from said antennaelements for producing a multiple digit code indicative of the directionof the incoming wave within a given one of a plurality of wide angularsectors, a second pair of spaced antenna elements spaced more widelyapart than the first pair, a second digital decision making meansresponsive solely to the comparative amplitudes of the signals from saidsecond pair of antenna elements for producing a second multiple digitcode indicative of the direction of the incoming wave within any one ofa plurality of narrower angular sectors than said wide angular sectors,and means responsive to said first and second digital codes fordetermining that one of the narrower sectors falling within the givenwide angular sector.

5. In the system of claim 4, an additional pair of antenna elementsspaced more widely apart than said second pair and an additional digitaldecision making means solely responsive to the comparative amplitudes ofthe signals from said additional pairsof elements for producing anadditional digital code indicative of the direction of the incoming wavewithin any one of a plurality of still narrower angular sectors thansaid narrow angular sectors, and means responsive to said first, second,and additional digital codes for determining that one of the stillnarrower angular sectors falling both within the narrower angular sectorand the wide angular sector,

6. in a direction finding system for an incoming wave, an antenna arraycomprising a plurality of pairs of spaced antenna elements, with eachpair being progressively spaced further apart than the preceding pair, adigital processing system for each pair of elements responsive solely tothe comparative amplitudes of signals from its associated pair ofelements, the digital processing system for each pair of elementsbeginning with the most closely spaced elements determining thepositioning of the incoming wave within progressively smaller angularsectors and each producing a multiple digit code indicative of saidpositioning, and means responsive to the digital codes from saidprocessing systems for determining the location of said incoming wave.

7. In a direction-finding antenna array, a pair of spaced antennaelements, a digital processing system responsive solely to theamplitudes of signals from said elements to produce a multiple digitcode indicative of the angular position of an incoming wave referencedto the common axis of said elements, said digital processing systemincluding means for frequency multiplying the signal from each element,and including means for adding and subtracting the amplitudes of saidfrequency multiplied signals and producing a digital code indicative ofwhether the added amplitude is greater or less than the subtractedamplitudes.

8. in the antenna array of claim 7, said digital processing meansfurther including means for adding and subtracting the amplitudes ofsignals from said antenna elements and producing a second digital codeindicative of whether the added amplitudes is greater or less than thesubtracted amplitudes.

9. In the antenna array of claim 7, said digital processing meansfurther including means for equally phase advancing and phase retardingthe signal from one of said elements, and including means for separatelyadding the signal from the other of said elements with each of the phaseadvanced and retarded signals, and additionally including means forcomparing the amplitudes of said separately added signals and producinga digital code indicative of the greater amplitude.

10. A digital direction finding system that is self-adjusting to a fortraveling waves comprising: a pair of spaced transducers, means forcombining the signals from said transducers to obtain a processed signalwhose amplitude varies according to the direction of the incoming wavereferenced to said transducers, and digitizing means for obtaining fromsaid processed signal a multiple digit code representative of thedirection of the incoming wave.

12. In the system of claim 11, a system of indicators energized by saidcode for substantially instantaneously indicating the direction of thewave.

13. In the system of claim 11, an additional pair of transducersoriented along a different axis than said first-mentioned pair, meansresponsive to said additional pair to produce a second digital coderepresenting the direction of the wave referenced to said additionalpair, and means correlating said first and second digital codes to moreprecisely determine the direction of the wave.

14. A direction-finding system comprising: an array of spacedtransducers, a plurality of independent modules each responsive tosignalsfrom a pair of transducers of said array to provide differentdigits of a digital code representing the direction of an incoming wavereferenced to the axis of said array, with one of the modules producingat least a two-digit code from a pair of transducers, and means forcorrelating the different digits to provide a code representing thedirection of the wave.

15. In the system of claim 14, means responsive to the frequency of thewave to translate the code number.

16. In the system of claim 14, each of the modules independentlyresponding to said signals whereby additional modules may be added todetermine the direction with greater definition,

17. In the system of claim 14, each of the modules comprising a meansfor combining a pair of signals to obtain a processed signal whoseamplitude varies according to the direction of the wave, receiver meansfor detecting the amplitude, and means for producing a digital coderepresenting direction.

18. A direction-finding system comprising: an array of spacedtransducers, a plurality of independent modules each responsive tosignals from said array to provide different digits of a digital coderepresenting the direction of an incoming wave referenced to the axis ofsaid array, and means for correlating the different digits to provide acode representing the direction of the wave, an additional array ofspaced transducers disposed along a different axis than said firstarray, a second plurality of independent modules each responsive tosignals from said array to provide different digits of a second digitalcode representing the direction of an incoming wave referenced to theaxis of said additional array, and means correlating said second codewith said first-mentioned code to determine the direction of the wavereferenced to both arrays.

1. A direction of finding system comprising: a pair of spaced antennas,and a rapid digital processing system responsive to the signals from theantennas to provide a digital code representative of the angle betweenan incoming wave referenced to a plane containing said pair of antennas,said digital processing system including a first decision making meansfor determining whether the sum of the signals from the antennas isgreater or less than the difference between said signals and producing adigital indication, and including a second decision making means,including phase-advancing and -retarding means for one of said signals,for determining whether the sum of the other signal and thephase-advanced signal is greater or less than the sum of the othersignal and the phase-retarded signal and producing a second digitalindication.
 2. A digital beam direction-finding system comprising: apair of spaced antenna elements, means for adding the signals from saidelements, means for differencing the signals from said elements, meansfor comparing the amplitude of said added signals with the amplitude ofsaid difference signals and producing a digital code indicative of thegreater amplitude, means for phase advancing the signals from one ofsaid elements, means for equally phase retarding the signal from thesame element, means for separately adding the signal from the otherelement with each of the phase-advanced and -retarded signals to producefirst and second processed signals, means for comparing the amplitudesof said first and second processed signals and producing a seconddigital code indicative of the greater amplitude, and means responsiveto the first and second digital code to determine direction.
 3. Adigital direction finding system having two displaced antenna elementsresponsive to an incoming beam comprising: means for comparing theamplitude of the sum of signals from said elements with the amplitude ofthe difference of the signals from said elements and producing a digitalcode indicative of the greater amplitude, means for equallyphase-advancing and -retarding the signal from one of said elements,means for separately adding the signal from the other element with eachof the phase-advanced and phase-retarded signal to obtain a first andsecond processed signal and producing an additional digital codeindicative of the greater amplitude, and means responsive to saiddigital code and additional digital code to determine the direction ofthe beam.
 4. A digital system for determining the direction of anincoming wave referenced to an antenna array comprising: a first pair ofspaced antenna elements in said array, digital decision making meansresponsive solely to the comparative amplitudes of the signals from saidantenna elements for producing a multiple digit code indicative of thedirection of the incoming wave within a given one of a plurality of wideangular sectors, a second pair of spaced antenna elements spaced morewidely apart than the first pair, a second digital decision making meansresponsive solely to the comparative amplitudes of the signals from saidsecond pair Of antenna elements for producing a second multiple digitcode indicative of the direction of the incoming wave within any one ofa plurality of narrower angular sectors than said wide angular sectors,and means responsive to said first and second digital codes fordetermining that one of the narrower sectors falling within the givenwide angular sector.
 5. In the system of claim 4, an additional pair ofantenna elements spaced more widely apart than said second pair and anadditional digital decision making means solely responsive to thecomparative amplitudes of the signals from said additional pairs ofelements for producing an additional digital code indicative of thedirection of the incoming wave within any one of a plurality of stillnarrower angular sectors than said narrow angular sectors, and meansresponsive to said first, second, and additional digital codes fordetermining that one of the still narrower angular sectors falling bothwithin the narrower angular sector and the wide angular sector.
 6. In adirection finding system for an incoming wave, an antenna arraycomprising a plurality of pairs of spaced antenna elements, with eachpair being progressively spaced further apart than the preceding pair, adigital processing system for each pair of elements responsive solely tothe comparative amplitudes of signals from its associated pair ofelements, the digital processing system for each pair of elementsbeginning with the most closely spaced elements determining thepositioning of the incoming wave within progressively smaller angularsectors and each producing a multiple digit code indicative of saidpositioning, and means responsive to the digital codes from saidprocessing systems for determining the location of said incoming wave.7. In a direction-finding antenna array, a pair of spaced antennaelements, a digital processing system responsive solely to theamplitudes of signals from said elements to produce a multiple digitcode indicative of the angular position of an incoming wave referencedto the common axis of said elements, said digital processing systemincluding means for frequency multiplying the signal from each element,and including means for adding and subtracting the amplitudes of saidfrequency multiplied signals and producing a digital code indicative ofwhether the added amplitude is greater or less than the subtractedamplitudes.
 8. In the antenna array of claim 7, said digital processingmeans further including means for adding and subtracting the amplitudesof signals from said antenna elements and producing a second digitalcode indicative of whether the added amplitudes is greater or less thanthe subtracted amplitudes.
 9. In the antenna array of claim 7, saiddigital processing means further including means for equally phaseadvancing and phase retarding the signal from one of said elements, andincluding means for separately adding the signal from the other of saidelements with each of the phase advanced and retarded signals, andadditionally including means for comparing the amplitudes of saidseparately added signals and producing a digital code indicative of thegreater amplitude.
 10. A digital direction finding system that isself-adjusting to different frequencies of an incoming wave comprising:a pair of spaced antenna elements, a digital processing systemresponsive solely to the amplitudes of signals from said elements toproduce a multiple digit code indicative of the angular position of anincoming wave referenced to the common axis of said elements, and meansresponsive to the frequency of the incoming wave for translating thedigital code to the correct code for the frequency received.
 11. Asubstantially instantaneous direction finding system for traveling wavescomprising: a pair of spaced transducers, means for combining thesignals from said transducers to obtain a processed signal whoseamplitude varies according to the direction of the incoming wavereferenced to said transducers, and digitizing means for obtaining fromsaid processed signal a multiple digit code representative of thedirection of the incoming wave.
 12. In the system of claim 11, a systemof indicators energized by said code for substantially instantaneouslyindicating the direction of the wave.
 13. In the system of claim 11, anadditional pair of transducers oriented along a different axis than saidfirst-mentioned pair, means responsive to said additional pair toproduce a second digital code representing the direction of the wavereferenced to said additional pair, and means correlating said first andsecond digital codes to more precisely determine the direction of thewave.
 14. A direction-finding system comprising: an array of spacedtransducers, a plurality of independent modules each responsive tosignals from a pair of transducers of said array to provide differentdigits of a digital code representing the direction of an incoming wavereferenced to the axis of said array, with one of the modules producingat least a two-digit code from a pair of transducers, and means forcorrelating the different digits to provide a code representing thedirection of the wave.
 15. In the system of claim 14, means responsiveto the frequency of the wave to translate the code number.
 16. In thesystem of claim 14, each of the modules independently responding to saidsignals whereby additional modules may be added to determine thedirection with greater definition.
 17. In the system of claim 14, eachof the modules comprising a means for combining a pair of signals toobtain a processed signal whose amplitude varies according to thedirection of the wave, receiver means for detecting the amplitude, andmeans for producing a digital code representing direction.
 18. Adirection-finding system comprising: an array of spaced transducers, aplurality of independent modules each responsive to signals from saidarray to provide different digits of a digital code representing thedirection of an incoming wave referenced to the axis of said array, andmeans for correlating the different digits to provide a coderepresenting the direction of the wave, an additional array of spacedtransducers disposed along a different axis than said first array, asecond plurality of independent modules each responsive to signals fromsaid array to provide different digits of a second digital coderepresenting the direction of an incoming wave referenced to the axis ofsaid additional array, and means correlating said second code with saidfirst-mentioned code to determine the direction of the wave referencedto both arrays.